Prove that: `sin(-420^0)(cos390^0)+cos(-660^0)(sin330^0)=-1.`

Explanation

To prove that \( \sin(-420^\circ) \cos(390^\circ) + \cos(-660^\circ) \sin(330^\circ) = -1 \), we will use trigonometric identities and properties of angles.

### Step 1: Simplify \( \sin(-420^\circ) \)
Using the identity \( \sin(-\theta) = -\sin(\theta) \):
\[ \sin(-420^\circ) = -\sin(420^\circ) \]

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